There is currently interest in the applications of diamond thin films for many different industrial fields. The primary reason is that diamond possesses many unique and superior properties which are unrivaled by other materials. These properties include high thermal conductivity, highly electrical insulating, extremely high hardness and a large refractive index. Diamond also has very low etching or corrosion rates by oxygen and under other harsh chemical conditions. Its potential for electronic applications has also been made clear (R. F. Davis and J. T. Glass, Advances in Solid-State Chemistry, JAI, London, 1991, vol.2). There is therefore a large demand for low-cost, high quality synthetic diamond. The lack of low-cost, high quality diamond has severely impeded the progress of applying the remarkable physical properties of diamond in electronic devices.
The recent development of diamond synthesis by low-pressure chemical vapor deposition (CVD) has given rejuvenated hopes for diamond applications. However, the realization of large-scale applications of diamond for electronics devices has been greatly hampered by the polycrystalline nature and surface roughness of CVD diamond. High quality synthetic diamond via homoepitaxial growth on single crystal diamond has been reported. Unfortunately, this approach to growing high quality diamond is economically impractical, as a low-cost single crystal diamond substrate of sufficient size is not readily available. Therefore, heteroepitaxial growth of diamond on a low-cost, readily available foreign substrate is a more viable approach and has been an important goal of the diamond researchers. As silicon is the most important material for the present microelectronics industry, diamond heteroepitaxy on silicon wafer has thus received much attention. However, the large lattice mismatch and the large difference in surface energy between silicon and diamond (lattice parameter of Si is 0.543 nm and surface energy of Si (111) plane is 1.5 Jm.sup.-2 ; while lattice parameter and surface energy of diamond are respectively 0.356 nm and 6 Jm.sup.-2) have raised considerable controversy about the feasibility of heteroepitaxial diamond growth on silicon substrates.
Recently, important advances towards heteroepitaxy of CVD diamond have been reported. By applying a negative bias voltage to the substrate during the nucleation stage in a MPCVD process, Stoner et al. (B. R. Stoner and J. T. Class, Appl. Phys. Lett. Appl. Phys. Lett., 60, 698, 1992; S. D. Wolter, B. R. Stoner, J. T. Glass, P. J. Ellis, D. S. Buhaenko, C. E. Henkins, and P. Southworth, Appl. Phys. Lett. 62, 1215, 1993) and Jiang et al. (X. Jiang and C. P. Klages, Diamond & Relat. Mater., 1, 195, 1992) obtained highly oriented diamond films on SiC and Si substrates. Substrate bias during CVD growth has now been established as an effective means for enhanced and oriented diamond nucleation. Although the mechanism of bias-enhanced epitaxial nucleation is still not well understood, experimental results have shown the important role of ion bombardment (X. Jiang, M. Paul, C. -P. Klages, and C. L. Jia, in Diamond Materials IV, 1995 Vol. 95-4, p. 50, S. McGinnes, M. Kelly, and S. B. Hagstrom, Appl. Phys. Lett. 66, 3117, 1995). Further, cross-sectional TEM study of the diamond films thus grown showed that local diamond heteroepitaxy grew directly on silicon without any interlayer (Jie Yang, Zhangda Lin, Li-Xin Wang, Sing Jin, and Zhe Zhang, Appl. Phys. Lett. 65, 3203, 1994; X. Jiang and C. L. Jia, Appl. Phys. Lett. 67, 1197, 1995). The result implies that a homogeneously clean silicon surface may be required for growing large-area diamond heteroepitaxy; a situation similar to molecular beam epitaxy.
Another relevant development concerning heteroepitaxial diamond nucleation is the present inventors recent results on the enhanced nucleation at low reactant gas pressure during CVD diamond growth (S. T. Lee, Y. W. Lam, Z. Lin, Y. Chen and Q. J. Chen, Phys. Rev. B., B55, 24, 1997). It is interpreted that enhanced nucleation at low pressure is due to a longer mean free path which gives rise to reduced collisions and thus a larger amount of diamond precursors arriving at the substrate. Further, at low pressure, the substrate surface is likely to remain cleaner, which is conducive to diamond heteroepitaxy.
In order to apply these important findings towards the production of large-area heteroepitaxial diamond films, two problems have to be addressed. The first one is on how to keep the chamber pressure low and still get enough ion bombardment. Another problem is that the ideal chamber pressure for nucleation is usually too low to allow for a high growth rate.